Supporting ``` as a code fence marker complicates the auto-fence
closing, and as per a discussion with Keegan on code-review@, it
is not worth the extra complexity.
(imported from commit 405afb95c4295a02f4677181456caf9d49913ac4)
And wire it up to our local copy of codehilite. This fixes highlighting in
fenced code blocks, e.g.
~~~~ .js
var x = function () {
return "hi";
};
~~~~
(imported from commit 0efb0c9b98a3acdf55e18bb1918af7960f3425be)